When a request is received for a template with a particular name, this should cause a query to be executed, which loads the relevant template content. This template content, together with the data model (the value of the 'user' variable in the examples above), should then be passed to FreeMarker.

However, the FreeMarker API seems to assume that each template name corresponds to a file of the same name within a particular directory of the filesystem. Is there any way I can easily have my templates loaded from the DB instead of the filesystem?

EDIT: I should have mentioned that I would like to be able to add templates to the database while the application is running, so I can't simply load all templates at startup into a new StringTemplateLoader (as suggested below).

Edit
You don't have to load all templates at startup. Whenever we will access the template, we'll fetch it from the DB and load it through the StringLoader and by calling template.process() we generate (in our case) the XML output.

Create a new implementation of TemplateLoader to load templates direct from the database, and pass it to your Configuration instance using setTemplateLoader() prior to loading any templates.

Use a StringTemplateLoader that you configure from your database when your application starts. Add it to the configuration as above.

Edit in light of the questioner's edit, your own implementation of TemplateLoader looks like the way to go. Check the Javadoc here, it's a simple little interface with only four methods, and its behaviour is well documented.